What is a Dungeon Starter?
Dungeon Starters are a form of GM prep for running the first session of a Dungeon World game. Dungeon Starters don’t dictate plot, they’re not Fronts (you write those after the first session), and they don’t replace the GM playbook. They are for those times when you want to just sit down and start a game right then. Dungeon Starters provide a unified flavor to your prep but, when you get down to it, they are really just an unordered cloud of blanks and hooks with some appropriate moving parts to make sure the players don’t catch you with nothing interesting to say. Dungeon Starters are made up of questions, impressions, custom moves, items and services, spells, and monsters (among other things).

Here are some that I've written:

Here are some that others have written:

How do you use one?
Lay out the playbooks for the players to choose from. While they’re making their selection, read through the Dungeon Starter to fill your mind with the flavor of the setting and get your creative juices flowing. Once players start to look like they’re finishing up with their character sheets, it’s time for the questions. You can totally ask the questions of the group but pointing to the players, one at a time, and asking them the questions individually can add some punch. Once you’ve asked a question, follow up with further questions and get players to present the character they’ve been working on. Once you’re ready to start playing, pull from the impressions and custom moves to frame scenes, fill them with the monsters and spells and things. Remember, players' answers are much more important than unspoken prep.

Why do they work?
So, how do you prep for a game where players can write in or out anything you might have prepared by the way they answer some questions? Dungeon starters work because they start out with a set of loaded questions. Those questions each establish facts about the setting that all the other details can hang on. If the first question asks, “What’s the worst thing about wading through Desanani swamps?” then you know you can plan some cool swamp leeches and write some moves about the swamp flies. You can make some haunted reed flutes and mud molding spells and be fairly sure you’ll have an opportunity to get them in the fiction because we already know the players are in a fantastisiful swamp. You won’t use all of the elements of a Dungeon Starter in play but you will have your choice of thematically appropriate elements to insert when you need them. You can also use Dungeon Starters over again and end up with a totally different story.

Writing your own...
Creating your own Dungeon Starters is a good way to prep for running the first session of your Dungeon World game. They steep your brain in fantasy and adventure and help you condense and refine your ideas. To write your own Dungeon Starter, start with the Blank Dungeon Starter sheet. Think about the kind of world you want to play in and the kinds of stories you want to tell there. I find that I get the best results when I come up with three general types of stories that I want to tell and then combine them. For example, you could write a mashup of sinking cities, the rivalries of ancient families, and a small 'e' exodus. Next, write down all the things that come to mind for each type of story and then pare it down to about 20 impressions – that seems to be the magic number. Because you'll be starting the adventurers off in media res, write questions that are loaded with the key setting details. Look to write questions that could lead to any of the impressions you wrote down and write at least one question for each player. Consider the types of things, services, spells, and monsters that could tie those stories together. Remember, everything you add to your Dungeon Starter is a chance to reinforce the themes and setting of your world. Write custom  moves to address local effects, stunts players might want to try, and situations you'd like to put them in. Use this opportunity to stretch the rules in interesting and unexpected ways. Finally, find or create some evocative art to flesh out your Dungeon Starter – not only is it a nice touch but you may find yourself drawing from the pictures in play.

A note on maps: I, for one, really like generating maps as part of play rather than prep, I may be alone in this. If you want to draw up some maps to go along with your dungeon starter, consider drawing 2-3 maps of specific rooms, buildings, or locations you can lay out for the players rather than the whole 'dungeon'.